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188                              CHEMICAL ENGINEERING
Air, hydrogen and methane are odorless, colorless, and tasteless and may be swallowed or inhaled with impunity. Chlorine has a strong, pungent, unbreathable odor, is greenish in color, and not only cannot be breathed or swallowed, but produces severe chemical burns, if brought in contact with the skin.
Laws Governing Behavior of Gases.From the above, it would be apparent that no two gases have exactly similar characteristics, and that no general rules can be laid down governing the handling of all gases in actual practice. There are, however, a number of laws governing the behavior of true gases under theoretical conditions, briefly as follows:
1.  Gases exert equal pressures on all parts of their containers and in all directions, and at all points within their mass.
2.  The volume of any gas varies inversely with the pressure, the temperature remaining constant (see p. 144).
3.  The volume of a gas, the pressure being constant, is proportional to its absolute temperature.
4.  The density of elementary gases is proportional to' their atomic weights.    The density of compound gases is proportional to one-half their molecular weights, hydrogen being one in both cases.
5.  A vapor not near its saturation point, that is, out of contact with its liquid, behaves in all respects like a true gas.
6.  The density of a saturated gas cannot be increased by increase in pressure or decrease of temperature.   Liquefaction will occur in consequence of such increase or decrease, which will prevent increase in pressure.
7.  The only gas which can prevent a liquid from evaporating is its own vapor.
8.  Mixtures of all gases exert pressure against the containing vessel equal to the sum of the individul pressures exerted by th e different component parts of the mixture.    The pressure exerted by any single gas is equal to the sum of the pressures of the different portions of the volume of that gas; that is, one-third of the gas in a container will exert one-third of the total pressure if that third is in the container alone.
9.  The flow of gases through an orifice is in inverse proportion to the square-root of the density of the gas; that is, hydrogen with a density of one, will escape four times as fast as oxygen with a density of 16, through the same orifice under similar conditions.
The above are the ordinary laws governing the behavior of gases. There are, however, variations from these rules. Theoretically the pressure-volume, or pv curve of any gas should be an equilateral hyperbola. As a matter of fact, as partially discussed on page 144, the pv curve of a number of gases varies slightly from an equilateral hyperbola. Methane at 300 Ib. shows a variation of as much as 4 per cent; other gases vary under low pressure and not at high, and vice versa. True pv curves can be plotted only from experiments; those for most of the industrial gases have been worked out by the various industries handling the gases.
The volume of a gas varies directly with changes in its temperature referred to absolute zero (-492F.), pressure remaining constant, and conversely the pressure varies directly as the temperature, the volume remaining constant. Roughly speaking, a variation of 5 (4.92) makes a difference of 1 per cent in the volume, or in the pressure of the gas.
Gases are Transported, Stored and Used, either Compressed, Liquefied or Absorbed.It may be said that any gas can be liquefied if all conditions of pressure and refrigeration are sufficient, and all gases are soluble to some extent in